How to clean up a mesh in order to export a freestyle vector for a 3D engraved lamp?
Go to the Object Data Properties tab, and under Remesh, set the mode to Quad and click Quadriflow Remesh

Set the target number of faces (I set 20,000 - eventually you will lose an undesired amount of mesh resolution, so don't go too low) and click the Ok button.

The result (at 20,000) looks like this:

The mesh looks even better at 10,000, but you will begin to get some artefacts if you get too low, so I suggest somewhere between 12,000 and 20,000 for your final result.